Enable Clang warning implicit-fallthrough and annotate the code.

BUG=4242
R=henrik.lundin@webrtc.org, stefan@webrtc.org, tommi@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/34899004

Cr-Commit-Position: refs/heads/master@{#8187}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8187 4adac7df-926f-26a2-2b94-8c16560cd09d
diff --git a/webrtc/modules/audio_coding/neteq/neteq_impl.cc b/webrtc/modules/audio_coding/neteq/neteq_impl.cc
index 4060a02..4dd8a6a 100644
--- a/webrtc/modules/audio_coding/neteq/neteq_impl.cc
+++ b/webrtc/modules/audio_coding/neteq/neteq_impl.cc
@@ -764,6 +764,7 @@
       sync_buffer_->IncreaseEndTimestamp(output_size_samples_);
       // Skipping break on purpose. Execution should move on into the
       // next case.
+      FALLTHROUGH();
     }
     case kAudioRepetition: {
       // TODO(hlundin): Write test for this.